In general, metallic materials having been manufactured are applied with rust preventive oil to avoid rusting until used at destinations where the materials are demanded (hereinafter referred to also as “to have initial rust resistance”). Typically, rust preventive oil is removed by degreasing treatment in advance of painting treatment, at such destinations. Meanwhile, the omission of degreasing treatment is advantageous in terms of environmental aspects related to effluent and industrial waste treatments and in terms of costs for management, chemicals, equipment and the like, and therefore steel sheets having excellent initial rust resistance even without rust preventive oil have been desired.
As a steel sheet having excellent initial rust resistance, for example, Patent Literature 1 describes a surface-treated steel sheet having on its surface a rust preventive film formed using Zn as a base, with the uppermost layer of the rust preventive film being an amorphous oxide layer composed of Zn, P and Si.
When painted, metallic materials are typically subjected to surface preparation treatment prior to painting, and resultant undercoats can enhance painting properties (corrosion resistance and adhesion of paint films).
The surface preparation treatment generally involves phosphate-based or zirconium-based chemical conversion treatment that follows a degreasing step, and when painting on constructs made of sheet metal, such as vehicle bodies, is assumed, such constructs are assembled after steps of cutting, pressing and joining of steel sheets, and then subjected to the surface preparation treatment. As with the degreasing step, the surface preparation treatment is desired to be omitted in terms of environment, costs and production efficiency.
Patent Literature 2 proposes an organic coated steel sheet that is excellent in initial rust resistance and lubrication with an organic coating alone, i.e., without application of rust preventive oil or lubricant oil, and is excellent in electrodeposition painting properties with the organic coating remaining on its surface, i.e., without removal of the coating by using, for instance, alkali and hot water.